This invention relates to the treatment of the waste stream from aluminum dissolution operations and, more particularly, to an improved method for regenerating the alkali etch solution and recovering aluminum hydroxide.
Treatment of aluminum articles of manufacture is carried out by such well known processes as etching, cleaning or chemical milling. Typically these processes involve the dissolution of aluminum metal according to the equation: EQU Al+NaOH+2H.sub.2 O.fwdarw.NaAlO.sub.2 +2.5H.sub.2 ( 1)
According to reaction (1), there is an increase in the concentration of the NaAlO.sub.2 and a decrease in the concentration of the alkali as the aluminum metal dissolves. However, the aluminate is not stable in water and, depending on existing conditions of temperature, concentrations and time, reacts with the water according to the following equilibrium equation: EQU NaAlO.sub.2 +2H.sub.2 O.revreaction.NaOH+Al(OH).sub.3 (b 2)
Theoretically, further additions of NaOH are required only to replace that which is physically attached to the work pieces removed from the bath. However, if the Al(OH).sub.3 is allowed to precipitate out in the etch bath, it is well known that the etch solution eventually becomes ineffective and unusable for carrying on the process and must be discarded and replaced. Attempts have been made to avoid the problems and waste of materials alluded to above.
In U.S. Pat. No. 4,372,805, there is shown a method for regenerating the sodium hydroxide wherein water is added to the solution containing dissolved aluminum to create a supersaturated solution of aluminum hydroxide, crystallizing the aluminum hydroxide, removing the same from the etch waste solution by centrifugation, and then recycling the remaining liquid to the etch tank. Examining equation (2) above, it would appear that the addition of water to the etch waste solution causes a shift of the equilibrium to the right in accordance with Le Chatelier's Principle, thereby causing the formation of increased aluminum hydroxide. However, that process is not completely satisfactory because the sodium hydroxide being recycled is diluted to such an extent that it is not sufficiently concentrated for use in the etching bath. In this regard, it is noted that the patent teaches the use of an evaporator in an effort to increase the concentration of the alkali.
In U.S. Pat. No. 4,136,026, there is shown another method wherein the etch waste solution is transferred first to a reactor vessel where it is apparently agitated to induce some precipitation of aluminum hydroxide. Some of the liquid from the reactor vessel is then transferred to a separator vessel where the aluminum hydroxide is separated from the solution with a vacuum drum filter. Due to the slow precipitation rate of aluminum hydroxide, the filter medium, as well as the filter cake, collects precipitate and problems of plugging soon occurred.
There thus exists a need for a more effective method of recovering and recycling the alkali from the etch waste solutions of aluminum dissolution operations.